1. Field of the Invention
The present invention relates to a small-sized image-taking lens system and, more particularly, to a high-performance compact image-taking lens system for a solid state imaging device, suitable for a digital input apparatus (such as a digital still camera or a digital video camera).
2. Description of the Related Art
In recent years, as personal computers and the like are being spread, a digital still camera, a digital video camera, and the like (hereinbelow, generically called digital cameras) capable of easily capturing image information into a digital device are being spread also among individual users. It is expected that such digital cameras will be spread more and more as image information input devices also in future.
The size of a solid state imaging device such as a CCD (Charge Coupled Device) mounted on a digital camera is also being reduced and, accordingly, a digital camera is also being requested to realize further reduction in size. Consequently, the image-taking lens system occupying large volume in a digital input device is also strongly demanded to be more compact. In miniaturization of the image-taking lens system, the easiest method is to reduce the size of a solid state imaging device. For this purpose, the size of a light receiving element has to be reduced, it becomes more difficult to manufacture the solid state imaging device, and the performance demanded for the image-taking lens system becomes higher.
On the other hand, when the size of the solid state imaging device is unchanged but the size of the image-taking lens system is reduced, the exit pupil position becomes inevitably closer to the image surface. When the exit pupil position becomes closer to the image surface, an off-axis light flux emitted from the image-taking lens system is incident obliquely on the image surface, the light collecting performance of a micro lens is not sufficiently exerted, and a problem occurs such that the brightness in the center part of an image and that in the peripheral part of the image become largely different from each other. If the exit pupil position of the image-taking lens system is disposed far from the image surface in order to solve the problem, increase in the size of the whole image-taking lens system is inevitable. In addition, due to increase in density of solid state imaging elements in recent years, the performance required of the image-taking lens system is also being increased.
Lens systems each comprised of four lenses for solid state imaging devices to address such demands are disclosed in Japanese Patent Laid-open Nos. 2002-228922, 2002-365529, 2002-365530, and 2002-365531, and U.S. Pat. No. 6,747,810.
In the configurations disclosed in Japanese Patent Laid-open Nos. 2002-228922, 2002-365529, 2002-365530, and 2002-365531, each of the four lenses is a single lens, so that there are many factors of causing a manufacture error accompanying a work of assembling the lenses, and degradation in the performance tends to be large.
In the configuration disclosed in U.S. Pat. No. 6,747,810, the second and third lenses are cemented to each other to obtain a three-group configuration, thereby reducing the manufacture error factors. However, since the aperture stop is disposed on the image surface side of the first lens, it is necessary to form the optical system largely to set the exit pupil position far from the image surface.